The invention relates to a projection exposure apparatus for microlithography comprising a measuring system, and to a method for measuring an optical element.
In order to operate a projection exposure apparatus economically, it is desirable to image mask structures onto a substrate in the form of a semiconductor wafer with an exposure time that is as short as possible, in order thus to achieve a throughput of exposed substrates that is as high as possible. This necessitates high radiation intensities for achieving a sufficient exposure of each individual one of the substrates. Particularly when using radiation in the ultraviolet or extreme ultraviolet (EUV) wavelength range, the influence of intensive radiation can have the effect that the temperature in the individual optical elements in the projection lens and the illumination optical unit of the projection exposure apparatus varies locally, which in turn influences the surface form and the refractive index of the optical elements. Temperature distribution and refractive index distribution can also vary temporally in the optical elements.
These radiation-induced changes in the properties of the individual optical elements can lead to deviations in the imaging behavior of the projection exposure apparatus. Changes in the properties of optical elements in the projection lens are manifested as aberrations of the projection lens. The extent of these aberrations depends on the radiation dose. Examples of effects of high radiation intensity in a lens element include densification and reduction of density of the lens element material. With the use of 193 nm radiation, a particularly great change in density is observed in quartz glass. Such effects are usually designated and also known as so-called “lens heating” effects.
The aberrations produced by lens heating change the imaging behavior of the projection lens generally continuously during the operation of the projection exposure apparatus. This detrimentally affects the quality of the lithographic imaging. In order to ensure a uniformly high imaging quality, it is conventional practice in many cases for the exposure operation of the projection exposure apparatus to be interrupted at regular time intervals in order to check the imaging behavior. However, such interruptions are costly since they result in a reduction of the throughput.